Photolithography is a critical step in semiconductor fabrication. Photolithography often includes the use of exposure and development in photoresist layers for forming photolithography patterns. However, the advances in integration level of chips require continuous decrease in feature size of photolithography process.
The resolution (R) of the exposure device determines the minimal feature size of the photolithography process. The resolution (R) of the exposure system or device may satisfy the relationship of R=kλ/(NA), where k represents a parameter associated with the exposure process, λ represents the wavelength of the light source used for exposure, NA represents the numerical aperture of the optical system in the exposure device. It can be shown from the relationship described above that, the resolution of the exposure device can be increased in two ways. One way is to increase the numerical aperture of the optical system, and the other way is to decrease the wavelength of the light source used for exposure.
Efforts have been made to increase the resolution by increasing the numerical aperture of the optical system. However, because the next-generation photolithography process may have considerably demanding requirements on the minimal feature size, a sufficiently large numerical aperture needs to be provided optically to satisfy the requirements. The large numerical aperture not only makes the photolithography device and the related modulation undesirably complex, but also greatly limits the depth of focus of the optical system.
Thus, the other way to increase the resolution, i.e., decreasing the wavelength of the light source used for exposure, has been studied. Extreme ultraviolet (EUV) light source is a newly developed light source. The wavelength of light for exposure, generated by the EUV light source, may be about 13.5 nm or even shorter. Applying the EUV light in the exposure system may obtain desirably small photolithography feature size.
In conventional technology, a main method used for generating EUV light is laser produced plasma (LPP). The working principle of the LLP includes using a laser source to generate a laser beam and using the generated laser beam to bombard tin (Sn) targets. The bombardment excites plasma, and the plasma radiates EUV light.
As shown in FIG. 1, the structure of a conventional EUV light source includes a tin droplet nozzle 101, a laser source 103, a lens unit 105, and a condenser mirror 107. The tin droplet nozzle 101 ejects tin droplets 102 downward intermittently. The laser source 103 is configured to generate a laser beam 104. After converged by the lens unit 105, the laser beam 104 bombards the tin droplets 102. The bombarded tin droplets 102 generate plasma, and the generated plasma radiates EUV light 108. The condenser mirror 107 is configured to collect the radiated EUV light 108 and converge the radiated EUV at the center of focus 109.
However, the EUV light generated by the conventional EUV light source has undesirably low power, which cannot meet the requirements of mass production. The disclosed device structures and methods are directed to solve one or more problems set forth above and other problems.